Biostatus and Distribution

This endemic moth is found throughout New Zealand where its host plants, Cordyline species (cabbage trees), are found. It is present in cities and native ecosystems.

Conservation status: Widespread, not threatened.

Life Stages and Annual Cycle

The moths are about 10 mm long with a wing span of 35 mm. Moths have pale brown wings. In the daytime, moths rest on dead leaves of cabbage trees. They sit at right angles to the long axis of the leaf with the wings stretched out and pressed flat against the leaf so that the narrow lines on the wings are in a line with the veins on the dead leaf. If disturbed, a moth will fly to and settle on another leaf, carefully aligning itself with the long, narrow leaf veins.

There have been no reports of moths mating. It is assumed that the newly emerged female moth emits a pheromone (a mixture of volatile chemicals), that attracts the male moth. Male moths have feathery antennae that have a large number of sensory cells for detecting chemicals in the air.

Eggs are laid in rows mainly on the underside of fronds. On dead leaves they are laid near the base. The eggs are green when first laid, but turn brown and then red. Eggs hatch after about 14 days.

Newly hatched caterpillars make their way to the unopened leaves of the crown. Their relatively flat shape enables them to get into the gap between leaves. The young caterpillars graze the surface of young leaves, leaving a long brown scar along the length of the leaf. When they are bigger, the caterpillars chew holes in and notches out of the sides of young leaves. The two kinds of damage to leaves are not seen until the leaves unfurl.

The caterpillars grow by changing skins, moulting. The small caterpillars are green, which makes them difficult to see, even if they are away from their normal protected sites. They appear to become darker as they grow bigger. Fully grown caterpillars reach 25 mm in length.

Caterpillar walking As a typical looper caterpillar, it has three pairs of legs by the head and two pairs of pseudo legs (false legs) at its rear end. The caterpillar walks by looping along. It first brings its rear end up towards its front and holds on with its false legs. Then it stretches forwards and holds on with its true legs while it again brings its rear end forwards, repeating the process. If disturbed, caterpillars may drop from the leaf suspended on a silk thread.

Cocoon and pupa When the caterpillar is fully grown, it spins a silk-lined cocoon in a sheltered site such as the base of dead leaves, in crevices in trees or in the litter on the ground. The caterpillar moults into a pupa that has a long hook at its rear end that holds the pupal skin in the cocoon, when the moth is emerging from the pupa. When moths emerge from their pupae, they crawl to a suitable place from which to hang. They expand their wings and then let them harden and dry.

Recognition

Adult moths have a distinctive appearance and may be found on dead leaves of their host plants, but may be difficult to see because of their good camouflage. Some moths will be resting on other plants and structures and may be more easily found.

The presence of cabbage tree moth caterpillars can be recognised by their distinctive damage to the young leaves of Cordyline species, cabbage trees. The caterpillars only damage the young soft leaves. This is seen when the leaves unfurl and expand. However, the damage lasts many years, as long as the leaves stay on the plant. The caterpillars are found in between the young leaves in the crown of the plant. It is the only looper caterpillar living in this part of the plant.

Two species of hoverflies, the large hoverfly, Melangyna novaezealandiae (Macquart, 1855) and Allograpta ropala (Walker, 1849) (Diptera: Syrphidae) have predatory larvae that feed on insects. Adult females lay eggs near potential prey. Hoverfly larvae would be able to go into the narrow spaces between the young leaves, especially those areas where larger caterpillars live.

Other insect predators may also feed on cabbage tree moth caterpillar. Birds have been observed searching the crowns of cabbage trees in a way that indicates they were looking for and feeding on cabbage tree moth caterpillars. Birds probably prey on the adult moths.

ParasitoidsThe endemic parasitoid flies, Pales feredayi (Hutton, 1901) and Pales nefaria (Hutton, 1901) (Diptera: Tachinidae) lay eggs on leaves, near caterpillars. Based on knowledge from other species, the caterpillar eats the fly egg. The parasitoid larva then hatches in the gut of the caterpillar. The fully grown fly larva kills the caterpillar after it has made its cocoon. The fly larva comes out of the caterpillar and pupates.

A recently established parasitoid wasp, Meteorus pulchricornis (Wesmael, 1835) (Hymenoptera: Braconidae), has also been recorded attacking cabbage tree moth caterpillars. The female parasitoid lays an egg in the caterpillar, which is later killed by the parasitoid larva.

Another parasitoid wasp has been reared from a caterpillar. Many wasp larvae emerged from the large caterpillar and spun white silk cocoons on the plant leaf. They pupated inside their individual cocoons. They are probably a species of Apanteles or Cotesia.

PathogenDavid Miller in his 1971 book mentions that colonies of caterpillars can be attacked by what he called a wilt disease. There have been no further reports of this disease although a caterpillar has been found infested by a dense white fungus.

Host Plants

The cabbage tree moth lives only on Cordyline species (Agavaceae), cabbage trees. Caterpillars and typical leaf damage have been found on five species. The young caterpillars graze the surface of young leaves, leaving a long brown scar along the length of the leaf. When they are bigger, the caterpillars chew holes in and notches out of the sides of young leaves. The two kinds of damage to leaves are not seen until the leaves unfurl.

Control

Cabbage tree moths are part of natural ecosystems and do not require control. However, heavy infestations on small, young plants in gardens may cause concern. The caterpillars live and feed between young leaves. This makes it very difficult to reach them with contact insecticides. If insecticides are to be used, they should be applied in the evening when caterpillars may leave their shelters. Choose products that cause least harm to natural enemies.

Additional Information

Camouflage Moths and caterpillars are good examples of camouflage. The moth not only looks like a dead cabbage tree leaf, but when it settles on a dead leaf, carefully aligns itself so that the pattern of lines on its wings matches that of the leaf veins on the dead leaf. The young caterpillars are green like the leaves. The caterpillars are also relatively flat so that they can hide between the young leaves.

Population fluctuations and monitoring changes Populations of cabbage tree moth caterpillars change greatly from year to year. Some years there are very high populations and the young leaves of most cabbage trees in an area are badly damaged, while in other years, such as 2009 in Auckland, there are few caterpillars and most cabbage trees have undamaged young leaves.

Because current season’s leaf damage does not show up until the leaves unfurl and expand, these changes are not immediately obvious. However, leaf damage could be used as a measure of cabbage tree moth populations. Standard measures of leaf damage would be required, and guidelines on which leaves to score. These would need to take into account that damage occurs on young leaves and that leaves may stay green for up to two years.

Bug Signs

Metal outdoor signs are available for placement in reserves, Regional and National parks, urban parks and school grounds. They can be bought from Metal Images Ltd, www.metalimage.co.nz/products/botanic-labels. The Bug Signs are listed near the bottom of the ‘Fauna Species list’. The signs come in two sizes, 100 x 200 mm, 194 x 294 mm. The signs can be bought ready mounted on a stand that needs to be ‘planted’ in the ground, or they can be bought unmounted with holes for fixing into your own mounts.

The signs for Cabbage tree moth are suitable for placement near any species of cabbage tree. Cabbage trees with low leafy shoots are particularly suitable because they usually show the typical leaf damage made by the caterpillar. Note that some years a cabbage tree leaves may be heavily damaged and other years there may be little damage.